Pretty much as everyone has explained already.
Can only remember bonking twice, by far the worst of which was in Cornwall during the 2003 heatwave. Hit the wall about 10 miles from the B&B and felt so bad I had to push the bike for a bit. Thankfully found a garage where I refuelled but I only felt marginally better. Felt absolutely horrible; dizzy, nauseus, hot/cold, weak, feverish, unable to think properly, etc. Spent the rest of the day recovering. May well have had some kind of heatstroke/exhaustion as well. Either way, it's not something you want to experience.
What made it worse was I dropped my Oakleys in a field (during the walking phase) and had to retrace my steps the next morning. Found them in the end.